Tissues in vivo have highly organized complex three-dimensional structures that comprise of heterogeneous cell populations and extra-cellular matrix (ECM). In mammals, the ECM can surround cells as fibrils, tubes and channels that contact the cells on all sides, or as a sheet called the basement membrane that cells ‘sit on’. The ECM is normally composed of proteins and polysaccharides to provide mechanical support and a biochemical barrier. A modified form of ECM, for example, appears in the form of bone. Research has shown that the functioning of cells is very much influenced by cell extracellular matrix. Some cells can only grow and prolong their cell survival in co-cultures of different cell types. However, co-cultures have been limited by the inability to manipulate or control the interaction of the different cells in the culture due to the lack of a suitable matrix for these cells.
Thus, because of the importance of a matrix for cell growth, it is a major goal of tissue engineering to recreate ECM structures that better mimic this matrix surrounding the cells in vivo, in particular to mimic the matrix of in vivo tissue.
The application of microtechnology in tissue engineering, which include micromachining, photolithography and sort lithography, has allowed the patterning of cells in size scales of micrometers relevant to the matrix of tissues. However, most work demonstrated previously was in creating two-dimensional patterns of single cells or multiple cell types, and few of three-dimensional patterning in which different layers are stacked one above the other to create an in vivo like three-dimensional structure for cells.
Wei Tan, M. S. and T. A. Desai (Biomaterials, 2004, Vol. 25, P. 1355-1364) describe reconstituted biopolymer matrices for the creation of three-dimensional patterns inside channels. Layer by layer micromolding in capillaries (MIMIC) via cell-matrix contraction has allowed the deposition of heterotypic cell layers in z-dimensions. Thereby, the biopolymer matrix is immobilized on the surface by binding the biopolymer matrix via a chemical linker to the surface of the carrier. However, this approach is limited to the patterning of one cell layer at a time and lacks precise control of the matrix surrounding the live cells.
In another approach of the same authors (Wei Tan, M. S. and T. A. Desai, Tissue Engineering, 2003, Vol. 9, No. 2, P. 255-267) native collagen and mixtures of collagen with chitosan or collagen, chitosan and fibronectin were used to create matrices for embedding human lung fibroblasts and human umbilical vein endothelial cells therein. Gelation took place in channels coated with BSA in which the polymer-cell mixture was pumped. However, long gelation times and missing means to control the gelation limit the use of this method in creating ECM like structures.
Another approach is the three-dimensional photolithography of hydrogels containing living cells (Liu and Bhatia, Biomedical Devices 2002, Vol. 4(4), P. 257-266). However, the cell toxicity of the photoinitiatior employed in photopolymerization is a major problem of this method.
Since the functionality of some kind of cells and the ability to grow depends to a great extent on the matrix surrounding the cell, there is still a need of effective systems for creating such matrices for different cell species.